The present invention relates to a method for forming a compound semiconductor film which is usable for a luminous element, solar battery, photosensor, such as a semiconductor laser, light-emission diode, electroluminesence (hereinafter called EL), and the like.
There have already been known such compound semiconductors as light-emission diodes, semiconductor lasers, and the like, for example, a GaAs thin film which, by the application of electric field thereto, converts electric energy to light energy.
A method for forming such a GaAs thin film includes a liquid phase growth method which comprises crystal growth from a component containing liquid phase directly on a substrate, and a gaseous phase growth method which comprises a reaction gas decomposition on a substrate to thereby deposit such a thin film thereon. However, the liquid phase growth method has the disadvantage that the temperature control for forming a liquid phase is difficult (therefore the film thickness control is difficult), and since a fusing agent is added to the phase, it is difficult to obtain a highly pure thin film. The gaseous phase growth method is also disadvantageous because due to the resultant decomposed product of a volatile compound and the unreacted gas no highly pure thin film is obtained and it is difficult to control the film thickness and the impurity doping. Besides, from the process engineering point of view, in both methods, there is a serious disadvantage in that it is impossible to uniformly form a thin film over a large area, so that these methods are not applicable to an industrial scale production.
On the other hand, there exist the following methods: the molecular beam epitaxis method which comprises evaporating a thin layer-forming material at a low rate under a ultra-high vacuum condition (e.g., 10.sup.-10 Torr) to attach onto a substrate to epitaxially grow thereon; the vacuum deposition method which comprises heating a component vapor source to evaporate the component into a vacuum space to deposit it on a substrate; the sputtering method which comprises causing charged heavy particles such as Ar.sup.+ ions to collide with a target consisting of a thin film-forming material to sputter the material therefrom and attaching it onto a substrate; and other equivalent methods.
In the above three methods a thin film-forming material evaporates without decomposing for deposition on a substrate, and they are fundamentally different from the foregoing gaseous growth method. These methods have the following advantages: In the molecular beam epitaxis method there occurs little contamination during the layer formation because of its high vacuum condition, and if necessary, the structure and the material of a semiconductor crystal can be changed. However, because this method requires a large scale, expensive apparatus, a vacuum deposition method or a sputtering method, whose apparatus structure is simple and inexpensive, is more advantageous from the standpoint of process engineering. However, it has now been found that even such vacuum deposition and sputtering methods have the following problems: Because the vacuum deposition method is normally operated under a vacuum condition of from 10.sup.-3 to 10.sup.-7 Torr and because the sputtering method is operated under a vacuum condition wherein the partial pressure of such an inert gas as Ar is of the order of from 10.sup.-1 to 10.sup.-3 Torr, The semiconductor crystal grows in an atmosphere with a relativey high concentration of residual gases (impurities) such as oxygen, carbon, etc., and in addition the film surface tends to react with these residual gases. Consequently, increasing the contamination in the crystal structure is unavoidable. For this reason, despite the excellent process engineering merits any practically satisfactory film quality has still not been obtained in the methods.